Many complex electronic circuits require various amplifier stages for a wide variety of applications. In many cases, it may be desirable to convert an input voltage to a current and then amplify the current before converting the current back to a voltage. This conversion from a voltage to a current at the input of the amplifier circuit permits the use of current mirrors.
A current mirror is a device comprising an input, an output and a common terminal that is typically connected to a power supply. The input may be connected to a current source. Ideally, the output current will then be equal to the input current and therefore the output is said to mirror the input current, thence the name. By varying the device properties, the mirror current may also be an amplified copy of the input reference current. This should ideally be true for varying loads at the output.
The use of current mirrors may frequently lead to decreased sensitivity to variations of parameters such as temperature or voltage supply. When the bias currents are small, it is often more economical to use current mirrors rather than resistors in order to save die area. Current mirrors comprised of active electronic circuit elements have been used extensively in analog electronic integrated circuits both as biasing elements and as load devices for amplifier stages. After the current mirror, the output current may be converted back to a voltage by means of a transimpedance amplifier.
In modern integrated circuit design, current mirrors are often designed using Metal-Oxide Semiconductor Field Effect Transistors (MOSFETs) or, less frequently, bipolar transistors. However, due to these active elements, the current mirror requires DC biasing power beyond what is required by the voltage-to-current and the current-to-voltage converters. The biasing power required by the current mirror may be significant and degrade the energy efficiency of the overall amplifier. The energy efficiency in integrated circuits in general, and for applications in portable electronics in particular, is of great importance. Energy efficiency directly impacts the battery lifetime of devices and heat dissipation may become a problem.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.